I have a 1989 LX Convertible 5.0 and the RPM gauge is stuck on 4000. It never moves unless you take the car above 4000 rpm's. It will then pick up at 4000 and go up, but only go back down to 4000 - its an auto. Any ideas about what is wrong with it? Thanks, this is my first post, although not my first mustang - I have two right now.